The present invention relates to a machine tool of the type comprising a machine stand and a changeover device for the exchanging of spindle heads fastened to a vertical carrier of the machine stand to perform treatment of a workpiece.
From DE 39 18 902 A1, a machine tool is known wherein the spindle head fastened to a vertical carrier can be exchanged. For this purpose, a changeover device is mounted to the machine stand, comprising two exchange arms pivotable by respectively 180.degree.. Each exchange arm can grip the upper end of a spindle head delivered from a magazine and, through a pivoting movement of 180.degree., position the spindle head adjacent the vertical carrier, so that the spindle head can be transferred to the vertical carrier to be locked thereon. The spindle head suspended on the exchange arm can be turned by 90.degree. relative to the exchange arm in such a manner that the tools projecting from the exchange arm, which in their operating position are oriented in the forward direction from to the machine stand, upon transfer to the spindle head magazine will project from the machine stand in a lateral direction. To allow a quick exchange of the spindle heads, two exchange arms are required, one exchange arm removing the spindle head from the vertical carrier while the other exchange arm supplies a new spindle head to the vertical carrier. Such an arrangement requires a high technical expenditure for the exchange arms. The spindle head magazine must extend to both sides of the machine stand and necessitates a complex transport mechanism.
Further, in a machine tool known from EP 0 204 151 B1, the spindle heads are suspended on a head exchange unit arranged on the machine stand for rotation about a vertical axis. The spindle heads are seized from above and pivoted to a position above the moving path of the vertical carrier which then will take over the spindle heads. This construction makes it necessary to provide a head exchange unit with a large turning radius on the machine stand, and the outwardly projecting spindle heads and tools require considerable space.
In a machine tool known from DE 92 02 245 U1, the machine stand is arranged next to a transfer station. The machine stand supports a vertical carrier which is movable in a vertical direction and is provided with an exchange arm pivotable about a horizontal axis. On the transfer station, two transfer sites are arranged above each other, each of them adapted to receive one spindle head. The exchange arm is laterally pivoted towards one of the two spindle heads and then pulls the spindle head horizontally to the vertical carrier. For exchange of the spindle heads, not only a vertical movement of the vertical carrier but also several reciprocating movements of the exchange arm must be performed. Additionally, it is required that the spindle heads at the transfer station and on the vertical carrier be guided in vertical guide bars, while a wide gap must be bridged for transfer of the spindle heads.